Medium voltage cables



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A was 1 N VEN TOR. 0/ 6/ 10 *15440/ BY United States Patent 3,541,228MEDIUM VOLTAGE CABLES Luigi Lombardi, Milan, Italy, assiguor to PirelliSocieta per Azioni, Milan, Italy, a corporation of Italy Filed May 20,1968, Ser. No. 730,657 Claims priority, application Italy, May 23, 1967,

Int. Cl. H01b 7/00 US. Cl. 174-120 Claims ABSTRACT OF THE DISCLOSUREBACKGROUND OF THE INVENTION Medium voltage cables, i.e., cables designedfor the range of about 1,000 to 30,000 volts are known. Generally theycomprise a conductor such as a copper wire or tube insulated withelastomeric materials, particularly cured polyolefin polymers,especially amorphous copolymers which may be saturated or unsaturated.The cables also include at least one semiconductive or screening layer.

These cables are often designed with an insulating layer disposed orsandwiched between two semiconductive layers in order to provide anequipotential surface to make the electric field of the insulatingmaterial uniform. The semiconductive layer forming the outer coveringcan be placed either by extruding the semiconductive composition on theconductor which is coated with at least the insulating layer, or byhelically winding a tape of semiconductive composition on the same unit.In the latter case the semiconductive composition is normally a tape offabric impregnated with conductive carbon particles and an adhesiveagent.

Medium voltage cables prepared by either of these measures have proveddeficient in that the semiconductive layer is merely superimposed on theinsulating layer and that there are hollow spaces between the two layerswhich give rise to loss of efliciency due to ionization phenomena.

Attempts to avoid these difiiculties by the use of adhesives to adherethe one layer to the other have been unsatisfactory because of thedifiiculties which arise in separating the layers when it is necessaryto join or splice one cable to another.

It has therefore long been a problem in the art to provide a mediumvoltage cable characterized by a close, intimate, contiguous surface tosurface contact between the insulating and semiconductive layers tothereby avoid losses due to ionization phenomena. The contact should benonadhesive so that separate cables can be readily spliced without undueditficulty.

THE INVENTION This invention provides medium voltage cables in which aconductor is insulated with a layer of a cured polyolefin copolymerinsulating material. The insulating layer is in a contiguousrelationship with at least one semiconductive layer which is derivedfrom a semiconductive composition comprising conductive carbon particlesdispersed in chlorosulfonated polyethylene. The insulating layer may beadjacent to the conductor and under the semiconductive layer, or it maybe sandwiched between two semiconductive layers, one of which isadjacent the conductor.

The term contiguous is used herein to define the relationship betweenthe layers. It means that there is close, intimate, nonadhesive contactbetween the surfaces of adjacent layers, and that there is substantiallyno air space between the surfaces. The contact between the adjacentsurfaces is a pressure contact similar to the contact achieved by thewell known process of force fitting a metal shaft in a disc. Because ofthe adhesive free contact between layers they can be readily separatedfor splicing.

It has been discovered that a contiguous relationship can be achievedbetween adjacent layers of cured, amorphous, saturated or unsaturated,polyolefin elastomerS and semiconductive compositions comprisingconductive carbon particles dispersed in a chlorosulfonated polyethylene polymer. This relationship is achieved by a heat treatmenteither subsequent to the placing of the layers in adjacent contact orwhile the layers are being placed in this relationship.

In one embodiment of the invention, which is preferred because foreconomy and efficiency, the semiconductive screen layer is placed overthe insulating layer by winding a tape of the semiconductive compositionobtained from a calendered sheet.

The insulating elastomeric materials utilized in the instant inventionare cured compositions which are either saturated or unsaturatedamorphous polyolefins. They are known insulating materials which aresubstantially linear, amorphous, high molecular weight copolymersobtained by copolymerization of ethylene with a-olefins containing oneor more double bonds. If the olefin is a diene the double bonds may beconjugated or unconjugated. Copolymerization is effected by knowntechniques. The most common a-Olefill used in the preparation of theseproducts is propylene. Unsaturated products are obtained from conjugatedor unconjugated dienes such as butadiene; isoprene; 2,3-dimethylbutadiene; 1,4-pentadiene; 2-methyl- 1,4-pentadiene; 1,5-hexadiene;2-methyl-1,5-hexadiene; 1,4-hexadiene; 1,4-heptadiene; 1,5-heptadiene;1,5-octadiene; dicyclopentadiene; Z-methylene 2,5 norbornene;4,7,8,9-tetrahydromethylindene; and similar compounds.

1 Such products are illustrated, for example, in Canadian Pat. No.703,930, which issued on Feb. 16,1965.

For use in this invention the insulating compositions are compounded bypreviously known methods with the appropriate curing or cross-linkingagents, accelerators, plasticizers, antioxidants, reinforcing fillersand the like. The composition may be extruded onto the base conductor oronto the conductor already coated with a semiconductive layer. They arethen cured as described in more detail below.

The chlorosulfonated polyethylenes used in the invention are knownproducts obtained by reacting polyethylone with chlorine and sulfurdioxide in accordance with known methods. Generally the polyethyleneutilized has a number average molecular weight of the order of about20,000. The sulfur content of the product is normally from about 1.2% to1.7%, and the chlorine content from about 26% to 40%. The use ofchlorosulfonated polyethylene as the carrier for the semiconductivecompositions is a special feature of this invention. In addition to thefact that it is possible by utilization of the polymer to effect theabove described contiguous relationship, the polymer also has excellentphysical and mechanical characteristics. This makes it possible toobtain calendered sheets of minimum thickness to be utilized in thepreparation of the winding tapes employed in the invention. Suchproducts are described, for example, at pages 387-390 of Polymers andResins, by Brage Golding, Van Nos- I trand co; Inc'., New York, 1959.

For the preparation of the semiconductive compositions, thechlorosulfonated polyethylene is compounded with the appropriateingredients, including the conductive particles, plasticizers such asbasic lead phthalate and stabilizers such as polyethylene glycol. Thecompounding procedures are in accordance with standard practice. Usefulcompositions suggested by way of example may contain, for example, fromabout 40 to 70 parts carbon black, to 20 parts basic lead phthalate and0.5 to 1 parts of polyethylene glycol per 100 parts of chlorosulfonatedpolyethylene. For the preparation of tapes the composition is calenderedto produce a sheet having a thickness of from about 0.1 to 0.5 mm. fromwhich the tapes can be prepared by cutting. I

The products of this invention may be prepared by any of a number ofmethods. All of them include a step in which a semiconductive layer incontact with an insulating layer is heated at a temperature of fromabout 100 C. to 200 C. for a sufficient period of time to elfect thecontiguous relationship. During this period the semiconductive materialmay seal to itself at overlapping points. The time of heating may varywithin a very wide range, and the best time will be selected on thebasis of the selected temperature, the dimensions of the cable, economyand ease of operation. Suitable heating periods will normally be withinthe range of from about 30 minutes to 3 hours.

In one procedure a tape of the semiconductive composition is helicallywound on a conductor previously coated with the cured polyolefinelastomer and the resulting product heated as described above. 7

Alternatively the semiconductive composition may be coated directly onthe conductor by extruding or by winding a tape, and the insulatingmaterial extruded onto the first layer. This product may be coated witha second semiconductive layer, and the completedunit heated to effectsimultaneous curing of the insulator and pressure fitting of thesemiconductive layers to the conductor and to the insulating layer.

In still another embodiment of the invention the semiconductivecomposition may be extruded directly on an insulated conductor at atemperature in the above defined range so as to obtain a semiconductivelayer in contiguous relationship with the insulating layer in oneoperation.

One very convenient procedure for heating the unit comprising theconductor and the insulating and semiconductive layers is to cover itwith a protective sheath in the form of a heat stable fabric such ascotton, rayon,

nylon or the like, or to cover it with a metallic sheath.

In either event the protected unit is' then conducted through a heatingzone such'as an oven to bring the temperature to the defined level. Theprotective sheath is then removed.

A permanent protective sheath can also be used. For example, the unitcould be coated with an elastomer such as polychloroprene, by any of theusual methods such as extrusion, and the protected unit then heated tocure the elastomer while at the same time bringing the semiconductivelayer into contiguous relationship with the insulating layer.

The invention may be better understood by reference to the attacheddrawing illustrating one aspect of the invention. In the drawing, 1 is aconductive cable, 2 is the insulating layer, and 3 is the semiconductivelayer.

What is claimed is:

1. A medium voltage electric cable comprising a conductor insulatedwith-an insulating layer of a cured amorphous olefine copolymer selectedamong the saturated and unsaturated amorphous olefine copolymers,'asurface of said insulating layer being in nonadhesive pressure contactwith an adjacent surface of a semiconductive layer comprising conductivecarbon particles dispersed in chlorosulfonated polyethylene, there beingsubstantially no hollow spaces between said insulating layer and saidsemiconductive layer.

2. Acable as in claim 1 in which the insulating layer is under thesemiconductive layer.

3. A cable as in claim -1 in which the insulating layer is sandwichedbetween two semiconductive layers.

4. A cable as in claim 1 in which the semiconductive layer is under theinsulating layer.

5. A cable as in claim 1 in which the semiconductive layer is a tapewinding.

References Cited UNITED STATES PATENTS 2,090,510 8/1937 Bower 174-121.2

3,049,584 8/1962 DAscoli 174120.2

3,098,893 7/1963 Pringle 174102.2

. 3,260,694 7 1966 Wang.

3,325,325 6/1967 Ward 174-110.44 X

703,930 2/ 1965 Canada.

E. A. GOLDBERG, Primary Examiner U.S. Cl. X.R. l17218; 174-102

